Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition, overlay and other parameters of nanoscale structures.
Ongoing reductions in feature size, increasing geometric complexity, and more diverse material compositions of semiconductor devices impose difficult requirements on optical metrology systems that are relied upon for process development and process monitoring.
Optical metrology systems must meet high precision and accuracy requirements for increasingly small metrology targets at high throughput (i.e., short move, acquire, and measure (MAM) times) to remain cost effective. In this context, measurement model compute times have emerged as a performance limiting issue in the design of optical metrology systems. More specifically, performing measurement model calculations with sufficient accuracy, particularly during high throughput operation (i.e., short MAM times) has become an important issue for optical metrology systems, particularly those systems offering large ranges of system parameter options.
In some examples, spectroscopic scatterometry measurements performed at multiple angles of incidence (AOI) and multiple azimuth angles have emerged in response to current metrology challenges. In some examples, these systems are configured rotating polarizer (RP) configurations and rotating polarizer, rotating compensator (RPRC) ellipsometry configurations. These systems offer ranges of available system parameters. For example, different values of azimuth angle, angle of incidence, illumination wavelength, and illumination polarization may be selected for particular measurements. These ranges of available system parameters are useful for increasing measurement sensitivity to parameters of interest and increasing measurement diversity that is useful for breaking correlations among parameters of interest.
Unfortunately, as metrology targets become more complex, so does the spectral polarization response of the metrology target. For example, the spectroscopic measurement of periodic targets sometimes results in large spectral variations and discontinuities. These measurement effects are sometimes termed grating anomalies, or Wood's anomalies. Some models of these grating anomalies are relatively simple, such as the reflection (or transmission) Rayleigh manifold. However, often these simplified models fail to sufficiently capture the observed grating anomalies present in spectroscopic measurements of current metrology targets. Other resonances triggered by the penetration of the illumination light into the grating structures themselves are visible as grating anomalies in the spectroscopic measurements. These resonances are difficult to incorporate directly into a measurement model. Thusfar, the regression of a measurement model capable of repeatably resolving parameters of interest from spectral data exhibiting grating resonance anomalies is far too computationally expensive for practical use. In some examples, simply excluding the spectral range where an anomaly arises causes regression results to suffer systematical errors. In extreme cases, this results in a failure of the metrology system to monitor the process.
The risk of triggering significant grating anomalies in the collected data increases when the measurement of a parameter of interest involves ranges of system parameter values such as azimuth angle, angle of incidence, illumination wavelength, and illumination polarization. Furthermore, the risks are highly dependent on parameters of the metrology target structure, such as pitch (period) in a 2-D grating and multiple grating pitches in different directions (e.g., orthogonal gratings in a 3-D grating structure).
Future metrology applications present challenges due to small feature size and multi-parameter correlation. Improvements to ellipsometer and reflectometer systems incorporating ranges of system parameter values such as azimuth angle, angle of incidence, illumination wavelength, illumination polarization, illumination Numerical Aperture (NA), and collection NA are desired.